The present invention relates to a CRT display apparatus, particularly relates to a CRT display apparatus having a normal-contrast-display mode and a high-contrast-display mode.
As described in Japanese Unexamined Patent Publication No. 3-167965, it is known to apply a video signal to not only a cathode but also a grid electrode within an electron gun of a CRT to display brighter images in a CRT display apparatus. FIG. 8 shows the structure of the video drive circuit of the CRT display apparatus disclosed in this Japanese Unexamined Patent Publication No. 3-167965.
In this figure, a video signal is input directly into a non-inverting amplifier 33 and an inverting amplifier 34 through an input terminal 34. The video signal of negative polarity output from the non-inverting amplifier 33 is amplified by a video output amplifier 35 while maintaining its polarity and thereafter applied to a cathode electrode 37 of a CRT 39. On the other hand, the video signal of positive polarity output from the inverting amplifier 34 is amplified by a video output amplifier 36 while maintaining its polarity and thereafter applied to a grid electrode 38 of the CRT 39. As described above, the CRT display apparatus attains high brightness display by applying video signals having different polarities to the cathode electrode and the grid electrode respectively.
FIG. 9 is a graph showing a relationship between an amplitude of an input video signal and a brightness of a screen in a display apparatus including a video drive circuit having a structure as described above and a CRT whose xcex3 value is 2.0. The x axis represents amplitude of an input signal and y axis represents brightness. In this graph, the brightness of the screen when an input video signal whose amplitude is at 100% is applied only to the cathode is defined as 100%. A curve C1 represents an amplitude-brightness characteristic when a video signal is applied only to the cathode, and a curve C2 represents an amplitude-brightness characteristic when the same video signal is applied to the cathode and the grid. It is apparent from this graph that, if the cathode and the grid are applied with the same video signal, the brightness is at 200% when the amplitude of this input video signal is at 100% which is twice the brightness (100%) in a case where only the cathode is applied with the video signal.
By using a video drive circuit having such a structure as described above, it is possible to double a maximum brightness. However, since the brightness increases for the whole range of the amplitude of an input video signal, a dark area corresponding to an input signal having an amplitude lower than 20% which is recognized to be at a gray level which is close to a black level if only the cathode is applied with an input video signal is not recognized to be at the gray level close to the black level if both of the cathode and the grid are applied with the input video signal because of increase of the brightness of this area. As a result, although the maximum brightness increases, the screen looks bleached as a whole, and accordingly the contrast is perceived to decline. Moreover, when the brightness of the dark area increases, noises within this area become conspicuous, whereby image quality is degraded.
To avoid the decline of contrast and the degradation of image quality due to the noises within the dark area, it has been proposed to shift a voltage (a video signal amplitude) applied to the grid by a predetermined value, for example, to shift the voltage applied to the grid to such an extent that the brightness of an area corresponding to an input video signal having a 15% amplitude when the cathode and the grid are applied with the same video signal (referred to as xe2x80x9ccathode+grid drivexe2x80x9d hereinafter) matches the brightness of the same area when only the cathode is applied with the video signal (referred to as xe2x80x9ccathode drivexe2x80x9d hereinafter).
In a graph of FIG. 10, a curve C3 represents an amplitude-brightness characteristic when only the cathode is applied with a video signal, a curve C4 represents an amplitude-brightness characteristic when both of the cathode and the grid are applied with the video signal, and a curve C5 represents an amplitude-brightness characteristic when both of the cathode and the grid are applied with the video signal and the voltage level of the grid is shifted (referred to as xe2x80x9ccathode+grid+level-shift drivexe2x80x9d hereinafter). In the graph of FIG. 10, input video signals are plotted within 0% to 30% amplitude range.
This graph shows that the brightness in the case of the cathode+grid+level-shift drive (C5) is about the same as the brightness in the case of the cathode drive (C3) so far as an input video signal is within the 10% to 20% amplitude range, however it falls below zero when the input video signal has an amplitude less than 7%. It means that the brightness of any area corresponding to an input video signal having 7% amplitude or less is all brought to the black level, which causes halftone-reproduction degradation.
As described above, the conventional video drive circuit which attains high-brightness display by applying a video signal to both of the cathode and the grid to increase the maximum brightness involves the problem of contrast decline due to brightness increase in the areas which are at the gray level and close to the black level and image quality degradation due to the noises in the dark areas. Even if the provision for shifting the level of the voltage applied to the grid is made to avoid such problems, it causes another problem of causing halftone-reproduction degradation in the dark areas.
Besides, since the conventional video drive circuit has two different signal lines for supplying an input video signal to the inverting amplifier and the non-inverting amplifier individually, stray capacitance due to wiring is large compared with the case of supplying an input video signal only to the non-inverting amplifier, and its frequency response is therefore degraded.
A first object of the present invention, which has been made addressing the above-described problems, is to provide a CRT display apparatus capable of displaying images in high contrast and high brightness without causing noises and halftone-reproduction degradation within dark areas. A second object of the present invention is to equip the CRT display apparatus capable of displaying images in high contrast and high brightness with ability to suppress frequency response degradation due to the stray capacitance.
The first object of the invention is achieved by a first display apparatus including a CRT having three cathode electrodes which are applied with an R-video signal, a G-video signal and a B-video signal respectively, and three G1 electrodes provided for the three cathode electrodes comprising:
a control signal generation circuit for generating an R-control signal, a G-control signal and a B-control signal by removing, from each of the R-, G- and B-video signals, portions which are below a predetermined level in amplitude respectively;
a control signal amplification circuit for inverting and amplifying the R-, G- and B-control signals output from the control signal generation circuit; and
a selection circuit for selecting, for each of the three G1 electrodes, either a corresponding one of the R-, G- and B-control signals inverted and amplified by the control signal amplification circuit or a potential of a predetermined value in accordance with an instruction from an outside, and applying each of the three G1 electrodes with a selected one of the corresponding one of the R-, G- and B-control signals and the potential.
In order to achieve the second object of the invention, the first display apparatus may further comprise a switch for connecting one end of a signal line the other end of which is connected to an input of the control signal generation circuit to an input terminal through which the R-, G- and B-control signals enter, the switch being opened while the selection circuit selects the potential and being closed while the selection circuit selects the R-, G- and B-control signals.
The first object is achieved also by a second display apparatus including a CRT having three cathode electrodes which are applied with an R-video signal, a G-video signal and a B-video signal respectively, and a common G1 electrode comprising:
a control signal generation circuit for generating a control signal by removing, from one of the R-, G- and B-video signals, portions which are below a predetermined level in amplitude;
a control signal amplification circuit for inverting and amplifying the control signal output from the control signal generation circuit; and
a selection circuit for selecting either the control signal inverted and amplified by the control signal amplification circuit or a potential of a predetermined value in accordance with an instruction from an outside, and applying the G1 electrode with a selected one of the control signal and the potential.
In order to achieve the second object of the invention, the second display apparatus may further comprise a switch for connecting one end of a signal line the other end of which is connected to an input of the control signal generation circuit to an input terminal through which the one of the R-, G- and B-control signals enters, the switch being opened while the selection circuit selects the potential and being closed while the selection circuit selects the control signal inverted and amplified by the control signal amplification circuit.
The first object is achieved also by a third display apparatus including a CRT having three cathode electrodes which are applied with an R-video signal, a G-video signal and a B-video signal respectively, and a common G1 electrode comprising:
a luminance signal generation circuit for mixing the R-, G-, and B-video signals in a predetermined ratio to generate a luminance signal;
a control signal generation circuit for generating a control signal by removing, from the luminance signal output from the luminance signal generation circuit, portions which are below a predetermined level in amplitude;
a control signal amplification circuit for inverting and amplifying the control signal output from the control signal generation circuit; and
a selection circuit for selecting either the control signal inverted and amplified by the control signal amplification circuit or a potential of a predetermined value in accordance with an instruction from an outside, and applying the G1 electrode with a selected one of the control signal and the potential.
In order to achieve the second object of the invention, the third display apparatus may further comprise a switch for connecting one end of each of signal lines the other end of each of which is connected to an input of the luminance signal generation circuit to an input terminal through which one of the R-, G- and B-control signals enters, the switch being opened while the selection circuit selects the potential and being closed while selection circuit selects the control signal inverted and amplified by the control signal amplification circuit.